Carbon nanotubes and production method thereof

ABSTRACT

The present invention relates to a method of producing carbon nanotubes, comprising a catalyst particle forming step of heating and reducing a catalyst raw material to form catalyst particles and a carbon nanotube synthesizing step of flowing a raw material gas onto the heated catalyst particles to synthesize carbon nanotubes, wherein a carbon-containing compound gas without an unsaturated bond is flowed onto the catalyst raw material and/or the catalyst particles in at least one of the catalyst particle forming step and the carbon nanotube synthesizing step.

TECHNICAL FIELD

The present invention relates to carbon nanotubes and a productionmethod thereof.

BACKGROUND ART

Carbon nanotubes are materials having cylindrical structures of rolledgraphene sheets and having one-dimensional structures with significantlylarge aspect ratios (see Non Patent Literature 1). It is known thatcarbon nanotubes have excellent mechanical strength, flexibility,semiconductive or metallic conductivity, and high chemical stability.Methods of producing carbon nanotubes such as arc discharge, laservaporization, and chemical vapor deposition (hereinafter referred to asCVD) have been reported. In particular, CVD is a synthesis method thathas received attention as a synthetic method suitable for large-quantitysynthesis, continuous synthesis, and high purification (see Non PatentLiterature 2).

In particular, it is verified that single-walled carbon nanotubes(hereinafter referred to as “SWCNTs”) exhibit metallic characteristicsor semiconductive characteristics depending on the manner of winding andthe diameter of the nanotube, and SWCNTs have been expected inapplications to electrical and electronic elements and the like. SWCNTshave been synthesized mainly by a catalytic CVD method of growingnanotubes (for example, see Non Patent Literature 3). The catalytic CVDmethod uses metal nanoparticles as a catalyst. While a gaseous carbonsource is being fed, the carbon source is pyrolyzed at a hightemperature to grow nanotubes from the metal nanoparticles as thecatalyst.

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: S. Iijima, Nature, 354, 56 (1991).-   Non Patent Literature 2: “Kabon Nanochubo no Kiso to Ohyoh (Basics    and Application of Carbon Nanotubes)” edited by Riichiro Saito and    Hisanori Shinohara, BAIFUKAN CO., LTD., 2004.-   Non Patent Literature 3: H. Dai, A. G. Rinzler, P. Nikolaev, A.    Thess, D. T. Colbert, and RE. Smalley, Chem. Phys. Lett. 260, 471    (1996).

SUMMARY OF INVENTION Technical Problem

Recently, mass production of multi-walled carbon nanotubes havingdiameters of about 10 to 20 nm has been developed, and several companieshave plants having production per year of about 100 tons; nowmulti-walled carbon nanotubes have been commercially available at about10000 yen/kg. In contrast, mass production of SWCNTs having excellentconductivity and flexibility has not been developed yet, and SWCNTsstill have sold at about several ten thousand yen/g, which is severalthousand times the price of the multi-walled carbon nanotubes.

To synthesize carbon nanotubes having small diameters, particularlySWCNTs, it is most important to form catalyst particles having diametersof several nanometers and hold the diameter during synthesis of carbonnanotubes. Unfortunately, in such catalyst particles having smalldiameters, much of chemically unstable surfaces thereof are exposed; forthis reason, these catalyst particles become coarse to reduce thesurface areas; or the catalyst particles readily react with oxygen orwater to be oxidized, so that the diameters of the carbon nanotubesbecome larger or crystallinity reduces.

Then, an object of the present invention is to provide a method ofproducing carbon nanotubes that can produce long carbon nanotubes havingsmall diameters and high crystallinity, and carbon nanotubes obtained bythe production method thereof.

Solution to Problem

The present invention provides a method of producing carbon nanotubes,comprising a catalyst particle forming step of heating and reducing acatalyst raw material to form catalyst particles, a carbon nanotubesynthesizing step of flowing a raw material gas onto the heated catalystparticles to synthesize carbon nanotubes, wherein a carbon-containingcompound gas without an unsaturated bond is flowed onto the catalyst rawmaterial and/or the catalyst particles in at least one of the catalystparticle forming step and the carbon nanotube synthesizing step. Themethod of producing carbon nanotubes can grow long carbon nanotubeshaving small diameters and high crystallinity at high density.

The present inventors think the following reason that the effect of thepresent invention is attained. Usually, under a heating atmosphere, thenumber of catalyst particles reduces and their particle sizes increasedue to high activity of the surfaces of the catalyst particles as timepasses, synthesizing carbon nanotubes having larger diameters andshorter lengths. In the present invention, in at least one step of acatalyst particle forming step of heating and reducing a catalyst rawmaterial to form catalyst particles and a carbon nanotube synthesizingstep of flowing a raw material gas onto the heated catalyst particles tosynthesize carbon nanotubes, a carbon-containing compound gas without anunsaturated bond is fed onto the catalyst raw material and/or thecatalyst particles. Then, carbon in the carbon-containing compound gaswithout an unsaturated bond is dissolved on the surfaces of the catalystparticles to form catalyst particles having carbon-containing regions ontheir surfaces. The catalyst particles having carbon-containing regionson their surfaces have stabilized surfaces, suppressing a reduction inthe number of particles and an increase in particle size. This can growlong carbon nanotubes having small diameters and high crystallinity. Areduction in the number of particles and an increase in particle sizeare suppressed as described above, so that carbon nanotubes can beproduced at high density. The method of producing carbon nanotubesaccording to the present invention are suitable for production of SWCNT.

In the present invention, it is preferable that the carbon-containingcompound gas without an unsaturated bond be flowed onto the catalystparticles at least in the carbon nanotube synthesizing step. If the rawmaterial gas and the carbon-containing compound gas without anunsaturated bond are flowed at least in the carbon nanotube synthesizingstep, an increase in the particle size of the catalyst particles issuppressed while carbon nanotubes are continuously growing, therebysuppressing an increase in the diameters of the growing carbonnanotubes. As a result, the carbon nanotubes to be synthesized havehigher crystallinity. An increase in the size of the catalyst particlesis suppressed more to increase the life of the catalyst, and longercarbon nanotubes can be synthesized.

It is preferable that the “carbon-containing compound gas without anunsaturated bond” be a saturated hydrocarbon gas and the saturatedhydrocarbon gas be methane. It is preferable that the raw material gascontain acetylene or a gas generating acetylene in the reactor.

In the method of producing carbon nanotubes according to the presentinvention, the carbon-containing compound gas without an unsaturatedbond is flowed onto the catalyst raw material and/or the catalystparticles at least in the catalyst particle forming step, and the methodmay comprise a releasing step of releasing the catalyst particles fromthe heated state between the catalyst particle forming step and thecarbon nanotube synthesizing step.

When the catalyst particles formed in the catalyst particle forming stephave the carbon-containing regions on their surfaces, the surfacesthereof are stabilized, so that the particle size or the like is barelychanged due to deactivation by oxidation caused by mixing of oxygenduring the process and changes in temperature. Usually, when thesurfaces of the catalyst particles are exposed and oxygen is mixed orthe temperature changes after production of the catalyst, the catalystreadily deactivates. In the present invention, the catalyst particleshave the carbon-containing regions on their surfaces; for this reason,for example, even if a catalyst production apparatus and a carbonnanotube synthesizing apparatus are separately provided, influences ofmixing of oxygen accompanied by conveyance between these apparatuses andchanges in temperature can be suppressed.

Moreover, the present invention is carbon nanotubes produced by themethod of producing carbon nanotubes, and provides carbon nanotubeshaving a ratio G/D of 10 or more determined by Raman spectroscopy.

Advantageous Effects of Invention

The present invention can provide carbon nanotubes having smalldiameters and high crystallinity, and a method of producing carbonnanotubes that can grow such carbon nanotubes long at high density.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating an outline of acatalyst-carrying support; FIG. 1( a) illustrates a case where thesupport is a particle, and FIG. 1( b) illustrates a case where thesupport is a fixed substrate.

FIG. 2 is a schematic view illustrating a production apparatus whencarbon nanotubes are produced by on-substrate thermal CVD.

FIG. 3 is a schematic view illustrating a production apparatus whencarbon nanotubes are produced by fluidized bed thermal CVD.

FIG. 4 shows SEM images of carbon nanotubes produced in Examples 1 and2.

FIG. 5 shows SEM images of carbon nanotubes produced in Examples 3 and4.

FIG. 6 shows Raman spectra of carbon nanotubes produced in Examples 1 to4.

FIG. 7 shows an SEM image of a carbon nanotube produced in ComparativeExample 1.

FIG. 8 shows SEM images of carbon nanotubes produced in VerificationExamples 3 and 4.

FIG. 9 shows Raman spectra of the carbon nanotubes produced inComparative Example 1 and Verification Examples 3 and 4.

FIG. 10 shows AFM images of a variety of catalyst-carrying substratescontaining catalyst-carrying substrates in Verification Examples 1 and2.

DESCRIPTION OF EMBODIMENTS

Suitable embodiments according to the present invention will now bedescribed in detail with reference to the drawings.

The method of producing carbon nanotubes according to the presentembodiment comprises a catalyst particle forming step and a carbonnanotube synthesizing step.

[Catalyst Particle Forming Step]

In the catalyst particle forming step, catalyst particles needed forsynthesis of carbon nanotubes are formed. The catalyst particles areformed by heating and reducing a catalyst raw material such as a metalfilm or a metal oxide film formed on a support with a reducing gas suchas hydrogen. At this time, an inert gas such as argon and nitrogen isused as a carrier gas.

It is preferable that a metal forming the catalyst particles be a metaltypically used in synthesis of carbon nanotubes, and contain one or moreelements selected from V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, and Au.Among these, Fe, Co, and Ni having a large amount of carbon solidsolution are particularly preferable.

The support carrying the catalyst particles varies according to themethod of synthesizing carbon nanotubes, and may be in a form of aparticle or in a form of a plate. It is preferable that the material forthe support contain one or more elements selected from the groupconsisting of Si, Al, Mg, Zr, Ti, O, N, C, Mo, Ta, and W. Specificexamples of the material include oxides such as SiO₂, Al₂O₃, and MgO;nitrides such as SiN₄ and AlN; and carbides such as SiC. Particularly, acomposite oxide of Al₂O₃—SiO₂ is preferable.

The support may include a carrier layer for carrying catalyst particles,and the catalyst particles may be carried on the carrier layer. The samematerial as that for the support can be used as the material for thecarrier layer. The support may have a function as a carrier layer, andin this case, the carrier layer is not necessarily carried.

Through the catalyst particle forming step, catalyst-carrying supports10 a and 10 b carrying catalyst particles on supports are obtained,respectively. As illustrated in FIG. 1( a), when a support 3 is in aform of a particle, the catalyst-carrying substrate 10 a is formed tohave a carrier layer 14 formed on the support 3 and spherical orsemi-spherical catalyst particles 15 formed on the carrier layer 14. Asillustrated in FIG. 1( b), when the support 3 is in a form of a plate,the catalyst-carrying substrate 10 b is formed to have sphericalcatalyst particles 15 embedded in the support 3.

For the method of adhering the catalyst raw material and the rawmaterial for the carrier layer onto the support, a support may beimmersed in a dissolution solution of these raw materials, or thedissolution solution of these raw materials may be applied onto asupport, and be dried. Physical deposition, sputtering, CVD, and thelike may be used.

It is preferable that the average particle size of the catalystparticles be 3 nm or less from the viewpoint of a reduction in thediameters of carbon nanotubes to be synthesized. It is preferable thatin the catalyst-carrying supports 10 a and 10 b, the density of thecatalyst particle be one or more particles/100 nm². Catalyst particleshaving a smaller particle size and a higher density can grow carbonnanotubes having smaller diameters at a higher density. Examples of themethod of measuring the average particle size of catalyst particlesinclude a method used in Examples described later.

It is preferable that the reducing temperature be 400 to 1000° C. It ismore preferable that the reducing temperature be 400 to 900° C. from theviewpoint of suppression of catalyst poisoning such as coking. When acarbon-containing compound gas without an unsaturated bond describedlater is flowed at the same time, it is still more preferable that thereducing temperature be 500 to 900° C. to well dissolve carbon in thecarbon-containing compound gas without an unsaturated bond on thesurfaces of the catalyst particles.

[Carbon Nanotube Synthesizing Step]

The catalyst particles formed in the catalyst particle forming step areheated, and a raw material gas is flowed onto the catalyst particles;thereby, carbon nanotubes can be synthesized.

Here, the “raw material gas” refers to a gas comprising a carbon sourcecontaining a carbon atom and a hydrogen atom and decomposed when heated,and is composed, for example, of a carbon source used in synthesis ofcarbon nanotubes and a carrier gas. Acetylene, ethylene, ethanol, andthe like can be used as the carbon source in the raw material gas.Acetylene may be contained in the raw material gas, or may be generatedin a reactor. It is preferable that the carrier gas contained in the rawmaterial gas be an inert gas such as argon and nitrogen. Alternatively,hydrogen may be used as the carrier gas. The “carbon-containing compoundgas without an unsaturated bond” described later is not included in theraw material gas.

In the carbon nanotube synthesizing step, the concentrations of thegases in the total amount of the gases to be fed into the reactor may beproperly adjusted by a conventional method to be optimized. For example,when the gas as the carbon source in the raw material gas is acetylene,it is preferable that the concentration in the reactor be 0.01 to 20% byvolume based on the total amount of gas to be fed into the reactor(including acetylene in the raw material gas and acetylene generated inthe reactor by decomposition), and it is more preferable that theconcentration be 0.1 to 5% by volume based on the total amount of gas tobe fed into the reactor. When only acetylene in the raw material gas isused of acetylene in the raw material gas and acetylene generated in thereactor by decomposition or the like, it is preferable that theconcentration be 0.01 to 15% by volume, and it is more preferable thatthe concentration be 0.1 to 2% by volume. The synthesized carbonnanotubes can be separated and recovered from the catalyst particles bythe conventional method.

It is preferable that the reaction temperature be 400 to 1000° C. Fromthe viewpoint of suppression of catalyst poisoning such as coking, it ismore preferable that the reaction temperature be 400 to 900° C. When thecarbon-containing compound gas without an unsaturated bond describedlater is flowed at the same time, it is still more preferable that thereaction temperature be 500 to 900° C. to well dissolve carbon in thecarbon-containing compound gas without an unsaturated bond on thesurfaces of the catalyst particles.

[Formation of Carbon-Containing Regions]

In the method of producing carbon nanotubes according to the presentembodiment, the carbon-containing compound gas without an unsaturatedbond is flowed onto the catalyst raw material and/or the catalystparticles in at least one of the catalyst particle forming step and thecarbon nanotube synthesizing step (carbon-containing region formingstep). Namely, the carbon-containing compound gas without an unsaturatedband may be flowed when the catalyst raw material such as a metal filmor a metal oxide film formed on the support is heated and reduced, orthe carbon-containing compound gas without an unsaturated bond may beflowed during synthesis of carbon nanotubes.

When the carbon-containing compound gas without an unsaturated bond isflowed onto the catalyst raw material and/or the catalyst particles,carbon in the carbon-containing compound gas without an unsaturated bondis dissolved on the surfaces of the catalyst particles to form catalystparticles having carbon-containing regions on their surfaces. Thesurfaces of the catalyst particles having carbon-containing regions ontheir surfaces are stabilized (protected), and a reduction in the numberof particles and an increase in particle size are suppressed. Thereby,long carbon nanotubes having small diameters and high crystallinity canbe grown. Because a reduction in the number of particles and an increasein particle size are suppressed as described above, carbon nanotubes canbe produced at high density. The method of producing carbon nanotubesaccording to the present invention is suitable for production of SWCNTs.

In particular, when the raw material gas and the carbon-containingcompound gas without an unsaturated bond are flowed on the catalystparticles at least in the carbon nanotube synthesizing step, an increasein the particle size of the catalyst particles is suppressed whilecarbon nanotubes are continuously growing, thereby suppressing anincrease in the diameters of the growing carbon nanotubes. As a result,the carbon nanotubes to be synthesized have higher crystallinity. Anincrease in the size of the catalyst particle is suppressed more toincrease the life of the catalyst, and longer carbon nanotubes can besynthesized.

It is preferable that the carbon-containing compound gas without anunsaturated bond be flowed in both of the catalyst particle forming stepand the carbon nanotube synthesizing step. If the carbon-containingregions are preliminarily formed on the surfaces of the catalystparticles before synthesis of carbon nanotubes, the particle size of thecatalyst particles can be reduced in advance. Even when thecarbon-containing regions are formed on the surfaces of the catalystparticles before synthesis of carbon nanotubes, the stable state of thesurfaces of the catalyst particles is not always kept until thesynthesis of carbon nanotubes is completed; for this reason, the rawmaterial gas and the carbon-containing compound gas without anunsaturated bond are also flowed onto the catalyst particles during thesynthesis of carbon nanotube to suppress an increase in the particlesize of the catalyst particles and an increase in the diameters ofcarbon nanotubes which are growing.

Through the specification, the “catalyst particles” refer to particlesformed by heating and reducing a catalyst raw material, such as a metalfilm or a metal oxide film, formed on a support with a reducing gas suchas hydrogen irrespective of whether the particles have thecarbon-containing regions on their surfaces or not.

Examples of the carbon-containing compound gas without an unsaturatedbond include saturated hydrocarbons, alcohols, amines, and ethers; amongthese saturated hydrocarbons are preferable. It is preferable that thecarbon-containing compound gas without an unsaturated bond have 2 orless carbon atoms, and it is particularly preferable that thecarbon-containing compound gas without an unsaturated bond have onecarbon atom. When the carbon-containing compound gas without anunsaturated bond is a saturated hydrocarbon, it is preferable that thesaturated hydrocarbon be methane. Carbon nanotubes will not grow fromthe carbon-containing compound gas without an unsaturated bond (seeFIGS. 10( a) to 10(f) described later). In contrast, if gases likeunsaturated hydrocarbons, which are more unstable than saturatedhydrocarbons and become stable when decomposed into carbon and a gassuch as hydrogen, are used, carbon nanotubes start growing, notattaining the effect of protecting the surfaces of the catalystparticles with carbon (see FIGS. 8( a) and 8(b) described later).

Whether carbon-containing regions are formed on the surfaces of thecatalyst particles can be checked by X-ray photoelectron spectroscopy(XPS). The effect of the present embodiment can be sufficientlyattained, for example, if the amount of carbon atoms observed by XPS is3 at % or more after etching with Ar. It is preferable that the value be3 to 100 at %, and it is more preferable that the value be 5 to 50 at %.

In formation of carbon-containing regions on the surfaces, it ispreferable that the concentration of the carbon-containing compound gaswithout an unsaturated bond be 0.1 to 50% by volume based on the totalamount of gases to be fed into a reactor. When the reducing temperatureand/or the reaction temperature is 900° C. or more in the catalystparticle forming step and/or the carbon nanotube synthesizing step,influences of catalyst poisoning such as coking readily occur; for thisreason, from the viewpoint of suppression of these influences, it ismore preferable that the concentration of the carbon-containing compoundgas without an unsaturated bond be 0.1 to 20% by volume.

When the carbon-containing compound gas without an unsaturated bond isflowed on the catalyst raw material and/or catalyst particles in thecatalyst particle forming step, the carbon in the carbon-containingcompound gas without an unsaturated bond is dissolved on the surfaces ofthe catalyst particles to form the catalyst particles havingcarbon-containing regions on their surfaces, as described above. In thiscase, a releasing step of releasing the catalyst particles from a heatedstate can be provided between the catalyst particle forming step and thecarbon nanotube synthesizing step.

When the catalyst particles have the carbon-containing regions on theirsurfaces, the surfaces are stabilized, so that the particle size or thelike is barely changed due to deactivation by oxidation caused by mixingof oxygen during the process and changes in temperature. Usually, whenthe surfaces of the catalyst particles are exposed and oxygen is mixedor the temperature changes after production of the catalyst, thecatalyst readily deactivates. In contrast, in the case of the catalystparticles having the carbon-containing regions on their surfaces, forexample, even if a catalyst production apparatus and a carbon nanotubesynthesizing apparatus are separately provided, influences of mixing ofoxygen accompanied by conveyance between these apparatuses and changesin temperature can be suppressed.

As a method of synthesizing long carbon nanotubes, addition of acatalyst activator such as water vapor during synthesis is known (forexample, Japanese Patent No. 4621896). The method of producing carbonnanotubes according to the present embodiment can be applied to a systemfor adding water vapor.

[Reactor]

The method of producing carbon nanotubes according to the presentembodiment can be performed in on-substrate thermal CVD or in fluidizedbed thermal CVD. The on-substrate thermal CVD includes hot-wall CVD toexternally heat a reaction tube and cold-wall CVD to heat only asubstrate while a reaction tube is kept at a low temperature, and any ofthese synthetic methods can be performed. FIG. 2 is a schematic viewillustrating a production apparatus for producing carbon nanotubes byhot-wall CVD. A reactor 21 includes a cylinder horizontally disposed andhaving one closed end, and includes a raw material gas feeding tube 25in communication between the outside of the container and the insidethereof. Heaters 24 are disposed around the reactor 21.

In the reactor 21, a support 23 having a catalyst raw material layeredthereon is placed on a quartz board 22, and is disposed inside thereactor 21. In this state, the catalyst particle forming step isperformed to form a catalyst-carrying support 10 b from the support 23.The catalyst-carrying support 10 b is heated, and a raw material gascontaining acetylene is flowed onto the catalyst-carrying support 10 bthrough the raw material gas feeding tube 25; then, carbon nanotubes canbe synthesized on the catalyst-carrying support 10 b. As the supportcarrying the catalyst, supports in plate forms as well as supports inpowder forms, bead forms, honeycomb forms, porous forms, fiber forms,tube forms, wire forms, net forms, lattice forms, sponge forms, andlayer forms can be used, for example.

In contrast, FIG. 3 is a schematic view illustrating a productionapparatus when carbon nanotubes are produced by fluidized bed thermalCVD. A reactor 1 vertically disposed includes a porous plate 2 disposedin a lower portion of the reactor 1; a raw material gas feeding tube 5for feeding a gas such as a raw material gas is connected to a furtherlower portion of the reactor 1 than the porous plate 2. The reactor 1 isfilled with a particulate support 3 on which a catalyst raw material islayered. Heaters 4 are provided so as to cover the outer periphery ofthe reactor 1. In this state, the catalyst particle forming step isperformed to form a catalyst-carrying support 10 a from the support 3.The catalyst-carrying support 10 a is heated, and a raw material gas orthe like is flowed onto the catalyst-carrying support 10 a through theraw material gas feeding tube 5 and holes of the porous plate 2; then,carbon nanotubes can be synthesized on the particulate support 3.

[Carbon Nanotubes]

The carbon nanotubes obtained by the method of producing carbonnanotubes according to the present embodiment have small diameters andhigh crystallinity. Catalyst particles having small particle size canexist on the support at high density, so that long carbon nanotubes areobtained at high density.

The diameters of carbon nanotubes can be determined with a transmissionelectron microscope (TEM), for example. The lengths (growth amounts) ofcarbon nanotubes can be determined with a scanning electron microscope(SEM), for example. Although an ideal diameter and length of a carbonnanotube depend on applications of carbon nanotubes, the method ofproducing carbon nanotubes according to the present embodiment canproduce relatively long carbon nanotubes having relatively smalldiameters.

The crystallinity of carbon nanotubes can be evaluated by Ramanspectroscopy. In the measurement by Raman spectroscopy, a G bandattributed to graphite structure is observed at or near 1590 cm⁻¹ and aD band attributed to crystal defects is observed at or near 1340 cm⁻¹.The carbon nanotubes having high crystallinity have low peaks in the Dband and high peaks in the G band. Namely, crystallinity is higher asthe value of the ratio of the intensity of G band to the intensity ofthe D band defined by the following Expression (1) (ratio G/D) ishigher:

ratio G/D=(G−Bg)/(D−Bg)  Expression (1)

where “G” indicates a peak top value of the G band, “D” indicates a peaktop value of the D band, and “Bg” indicates a background correctionvalue which is an average of the values from 600 cm⁻¹ to 1000 cm⁻¹.

It is thought that in the carbon nanotube having a low ratio G/D,amorphous carbon and the like adhere to the surfaces thereof, and theproportion of non-linear and bent carbon nanotubes is high. Amorphouscarbon adhering to the surfaces of the carbon nanotubes is likely toinhibit electric conductivity between the carbon nanotubes, and bentcarbon nanotubes have reduced electric conductivity and mechanicalstrength. For this reason, to draw electrical properties and mechanicalstrength of the carbon nanotubes, it is preferable that the ratio G/D be8 or more, and it is more preferable that the ratio G/D be 10 or more.

As above, suitable embodiments according to the present invention havebeen described, but the present invention will not be limited to theseembodiments.

EXAMPLES

The present invention will be described more specifically by way ofExamples and Comparative Examples. The present invention will not belimited to these examples below.

Example 1 Formation of catalyst-carrying substrate

For a support, a silicon substrate with a thermally-oxidized film wasused. On the silicon substrate, metal aluminum film (thickness: 15 nm)as a carrier layer, and a metal iron film (thickness: 0.7 nm) as acatalyst raw material were sequentially formed by sputtering.

(Formation of Catalyst Particles)

Next, the substrate was placed inside the reactor illustrated in FIG. 2,and catalyst particles were formed. The total flow rate of a gas to beintroduced was 500 sccm (standard cubic centimeter per minutes), and thegas included hydrogen (25.0% by volume) and methane (1.0% by volume) ascomponent gases; and argon was used as an atmosphere gas. The innertemperature of the reactor was 800° C., and the reaction time was 5minutes. Thus, a catalyst-carrying substrate was obtained. The valueexpressed with % by volume based on the total amount of gases to be fedto the reactor.

(Synthesis of Carbon Nanotubes)

Next, acetylene (0.15% by volume) was additionally introduced withoutchanging the total flow rate, and carbon nanotubes were synthesized. Asother gases, hydrogen (25.0% by volume) and methane (1.0% by volume),and argon as an atmosphere gas were introduced. The inner temperature ofthe reactor was still 800° C. from formation of the catalyst particles,and the reaction time was 10 minutes.

(Evaluation of Amount of Growth of Carbon Nanotubes)

The produced carbon nanotubes were observed with a scanning electronmicroscope (SEM, available from Hitachi, Ltd.: S-4800), and the resultis shown in FIG. 4( a). The carbon nanotubes grew 710 μm from thesurface of the silicon substrate. The synthesized carbon nanotubes wereobserved with a transmission electron microscope (TEM, available fromJEOL, Ltd.: 2000-EX) to measure the diameters of any 36 carbonnanotubes; the average diameter was 2.6 nm.

(Raman Measurement)

The crystallinity of the synthesized carbon nanotubes was evaluated witha Raman spectrometer (HORIBA Ltd.: HR-800) by Raman spectroscopy. Thewavelength for measurement was 488 nm. As a result of the measurement,as shown in FIG. 6( a), a G band attributed to graphite structure wasobserved at or near 1590 cm⁻¹ and a D band attributed to crystal defectswas observed at or near 1340 cm⁻¹. The ratio G/D indicatingcrystallinity was 11.1 from the ratio of the intensity of the G band tothat of the D band, which indicated high crystallinity.

Example 2

Carbon nanotubes were produced in the same manner as in Example 1 exceptthat the amounts of hydrogen and methane to be fed were changed as inTable 1. The produced carbon nanotubes were observed in the same manneras in Example 1 with an SEM, and the result is shown in FIG. 4( b). Thecarbon nanotubes grew 670 μm from the surface of the silicon substrate.The measurement was performed in the same manner as in Example 1 byRaman spectroscopy. As a result, it turned out that the ratio G/D was10.4; and the ratio G/D of 10 or more indicated high crystallinityalthough it was not as high as in Example 1 (FIG. 6( b)).

Example 3

Carbon nanotubes were produced in the same manner as in Example 1 exceptthat the amounts of hydrogen and methane to be fed were changed as inTable 1. The produced carbon nanotubes were observed in the same manneras in Example 1 with an SEM, and the result is shown in FIG. 5( a). Thecarbon nanotubes grew 550 μm from the surface of the silicon substrate.The measurement by Raman spectroscopy was performed in the same manneras in Example 1. As a result, the ratio G/D was 10.4; and the ratio G/Dof 10 or more indicated high crystallinity although it was not as highas in Example 1 (FIG. 6( c)).

Example 4

Carbon nanotubes were produced in the same manner as in Example 1 exceptthat water vapor was added to activate the catalyst as shown in Table 1.The produced carbon nanotubes were observed in the same manner as inExample 1 with an SEM, and the result is shown in FIG. 5( b). The carbonnanotubes grew 1170 μm from the surface of the silicon substrate. Themeasurement by Raman spectroscopy was performed in the same manner as inExample 1. As a result, the ratio G/D was 12.5, which indicated highcrystallinity (FIG. 6( d)). This indicates that the effect ofintroducing methane is also effective in the system to which thecatalyst activator is added.

Comparative Example 1

Carbon nanotubes were produced in the same manner as in Example 1 exceptthat the amounts of hydrogen and methane to be fed were changed as inTable 1. The produced carbon nanotubes were observed in the same manneras in Example 1 with an SEM, and the result is shown in FIG. 7. Thecarbon nanotubes grew 440 μm from the surface of the silicon substrate,and it turned out that the amount of growth of the carbon nanotubes wassmaller than those in Examples 1 to 3. The diameters of any 33 carbonnanotubes were observed in the same manner as in Example 1 with a TEM,and measured an average of 3.4 mm. The measurement by Raman spectroscopywas performed in the same manner as in Example 1. As a result, the ratioG/D was 7.2, which indicated crystallinity inferior to those in Examples1 to 3 (FIG. 9( a)).

TABLE 1 Carbon nanotube synthesizing step Carbon Catalyst particleforming step source gas Saturated Saturated in raw Evaluationhydrocarbon hydrocarbon material Length RATIO Diameter H₂ gas H₂O H₂ gasgas H₂O (μm) G/D (nm) Example 1 25 vol % CH₄ 1.0 vol % 0 ppmV 25 vol %CH₄ 1.0 vol % C₂H₂ 0.15 vol % 0 ppmV 710 11.1 2.6 Example 2 26 vol % — 0ppmV 25 vol % CH₄ 1.0 vol % C₂H₂ 0.15 vol % 0 ppmV 670 10.4 — Example 325 vol % CH₄ 1.0 vol % 0 ppmV 26 vol % — C₂H₂ 0.15 vol % 0 ppmV 550 10.4— Example 4 25 vol % CH₄ 1.0 vol % 50 ppmV 25 vol % CH₄ 1.0 vol % C₂H₂0.15 vol % 50 ppmV 1170 12.5 — Comparative 26 vol % — 0 ppmV 26 vol % —C₂H₂ 0.15 vol % 0 ppmV 440 7.2 3.4 Example 1 * “vol %” indicates % byvolume based on the total amount of gases fed to the reactor. If thetotal is less than 100 vol %, the rest thereof constitutes argon. *“ppmV” indicates a unit for expressing ppm as a volume concentration.

Verification Examples 1 and 2

The catalyst-carrying substrate obtained through the steps in Example 4was used as a catalyst-carrying substrate in Verification Example 1.Formation of a film and catalyst particles was performed in the samemanner as in Example 2 except that water vapor was added to activate thecatalyst as shown in Table 2, and a catalyst-carrying substrate inVerification Example 2 was obtained.

TABLE 2 Catalyst particle forming step Evaluation Hydrocarbon LengthRATIO H₂ gas H₂O (μm) G/D Verification 25 vol % CH₄ 1.0 50 ppmV Not grow— Example 1 vol % Verification 26 vol % — 50 ppmV Not grow — Example 2Verification 25 vol % C₂H₄ 1.0 50 ppmV 80 6.0 Example 3 vol %Verification 25 vol % C₂H₂ 1.0 50 ppmV 32 2.4 Example 4 vol % “vol %”indicates % by volume based on the total amount of gases fed to thereactor. If the total is less than 100 vol %, the rest thereofconstitutes argon. “ppmV” indicates a unit for expressing ppm as avolume concentration.

(Observation of Catalyst Particles with AFM)

To observe the shapes of the catalyst particles immediately before thesynthesis of carbon nanotubes, the surfaces of the catalyst-carryingsubstrates in Verification Example 1 and Verification Example 2 afterthe catalyst particle forming step were observed. To examine thetendency attributed to the difference in the thickness of the catalystraw material, the substrates having thicknesses of metal iron of 0.7 nm(Verification Example 1 and Verification Example 2), 0.5 nm, and 1.0 nm(as values before and after 0.7 nm) were observed in the same manner.The observation was performed with an atomic force microscope (AFM;available from SHIMADZU Corporation: SPM-9600) on the followingmeasurement conditions: the scanning region was 200 nm×200 nm, thescanning rate was 1 Hz, and the number of pixels was 512×512.

The results when the catalyst-carrying substrate in Verification Example1 was contained and methane was introduced in the catalyst particleforming step are shown in FIG. 10( a) to FIG. 10( c). The results whenthe catalyst-carrying substrate in Verification Example 2 was containedand methane was not introduced are shown in FIG. 10( d) to FIG. 10( f).In the drawings, white (bright) spots are catalyst particles; no fibroussubstance was found on the surface of the substrate when methane wasintroduced, and it was found that carbon nanotubes did not grow.

The images were analyzed to evaluate the density of catalyst particlesand the size of the catalyst particle (average value), and the resultsare shown in Table 3. The size of the catalyst particle was calculatedfrom the density of catalyst particles and the amount of the film formedby sputtering, assuming that the shape of the catalyst wassemi-spherical. It turned out that when methane was introduced, the sizeof the catalyst particle was smaller and the density of catalystparticles was higher than those when methane was not introduced. Namely,the density of catalyst particles is one or more particles in a regionof 10 nm×10 nm when methane is introduced while the density of catalystparticles is one or less particle in a region of 10 nm×10 nm whenmethane is not introduced (low density). Consequently, it turned outthat introduction of methane suppresses an increase in the size of thecatalyst particles.

TABLE 3 Thickness of iron formed into a film 0.5 nm 0.7 nm 1.0 nmVerification Density of catalyst 1.42 1.30 1.48 Example 1 particles(particle/100 nm²) Size of catalyst 2.56 2.95 3.18 particle (nm)Verification Density of catalyst 0.85 0.90 0.87 Example 2 particle(particle/ 100 nm²) Size of catalyst 3.04 3.34 3.80 particle (nm)

(Analysis of Surface State of Catalyst by XPS)

To evaluate the element composition on the surface of the substrateimmediately before the synthesis of carbon nanotubes, the surfaces ofthe catalyst-carrying substrates in Verification Example 1 andVerification Example 2 were analyzed after the catalyst particle formingstep. The analysis was performed with an X-ray photoelectronspectrometer (XPS; available form ULVAC-PHI, Inc.: PHI 5000VersaProbeII). In the measurement, monochromatized AlKα-rays of 1486.6keV were used, the is peak top of C for charge correction was 284.8 keV.The range in the measurement was 200 μm. The angle of detection was 45°from the surface of the sample. Impurities in the air adhere to thesurface of the substrate; for this reason, the surface thereof wasetched with Ar, and the element compositions detected before and afterthe etching were analyzed. The results of analysis were shown in Table4. In the etching, a voltage of 1000 V was applied for 20 seconds, andthe etching was performed such that the thickness of an SiO₂ film wasabout 1 nm. As a result, it was found that compared to thecatalyst-carrying substrate in Verification Example 2 to which methanewas not introduced, a large amount of C existed on the surface of thecatalyst after the etching and the surface of the catalyst was coveredwith carbon in the catalyst-carrying substrate in Verification Example 1to which methane was introduced.

TABLE 4 Total Detected elements (at %) C O Fe Al (at %) VerificationBefore etching 44.3 37 1.8 16.8 100 Example 1 with Ar After etching 29.344.3 3.7 22.7 100 with Ar Verification Before etching 15.8 55.7 3.1 25.4100 Example 2 with Ar After etching 2.6 62.8 4.7 29.9 100 with Ar

Verification Example 3

Formation of a film and catalyst particles was performed in the samemanner as in Example 4 except that ethylene (1.0% by volume) was usedinstead of methane (1.0% by volume) as a component gas; acatalyst-carrying substrate in Verification Example 3 was obtained.

When the substrate was visually observed, it was found that the surfaceof the central portion of the substrate was slightly black. Next, thecenter of the substrate was observed in the same manner as in Example 1with an SEM. As a result, it turned out that the carbon nanotubes grewslightly (about 80 μm) as shown in FIG. 8( a), and the catalystparticles did not have the states of surfaces as in VerificationExample 1. The measurement was performed in the same manner as inExample 1 by Raman spectroscopy. As a result, it turned out that theratio G/D indicating crystallinity was 6.0, and crystallinity was poor(ratio G/D: 10 or less) (FIG. 9( b)). From this, it is thought that thesize of the catalyst particles increased before the carbon nanotubesynthesizing step was performed.

Verification Example 4

Formation of a film and catalyst particles was performed in the samemanner as in Verification Example 3 except that ethylene in VerificationExample 3 was changed to acetylene, and a catalyst-carrying substrate inVerification Example 4 was obtained. When the substrate was visuallyobserved, it was found that the center of the substrate was slightlyblack. Next, the central portion was observed in the same manner as inExample 1 with an SEM. As a result, it turned out that the carbonnanotubes grew slightly (about 32 gym) as shown in FIG. 8( b), and thecatalyst particles did not have the states of surfaces as inVerification Example 1. The measurement was performed in the same manneras in Example 1 by Raman spectroscopy. As a result, it turned out thatthe ratio G/D was 2.4, and crystallinity was poor (ratio G/D: 10 orless) (FIG. 9( c)). From this, it is thought that the size of thecatalyst particles increased before the carbon nanotube synthesizingstep was performed.

INDUSTRIAL APPLICABILITY

The method of producing carbon nanotubes according to the presentinvention can mass-produce long carbon nanotubes having small diametersand high crystallinity and can reduce production cost significantly.Accordingly, it is noteworthy that the carbon nanotubes produced by themethod according to the present invention are used in transparentelectrodes, semiconductor thin films, materials for electrodes inlithium ion batteries, materials for electrodes in fuel cells, materialsfor electrodes in electric double-layer capacitors, filler materials forcomposite polymers, electron emission guns, field emission display,probes for microscopes, gas absorbing materials, and the like. Inparticular, it is noteworthy that the SWCNTs produced by the methodaccording to the present invention are used in transparent electrodes,materials for electrodes in lithium ion batteries, materials forelectrodes in electric double-layer capacitors, and the like.

REFERENCE SIGNS LIST

1, 21 . . . reactor, 2 . . . porous plate, 3, 23 . . . support, 4 . . .heater, 5, 25 . . . raw material gas feeding tube, 10 a, 10 b . . .catalyst-carrying support, 14 . . . carrier layer, 15 . . . catalystparticle, 22 . . . quartz board, 24 . . . heater.

1. A method of producing carbon nanotubes, comprising: a catalystparticle forming step of heating and reducing a catalyst raw material toform catalyst particles, and a carbon nanotube synthesizing step offlowing a raw material gas onto the heated catalyst particles tosynthesize carbon nanotubes, wherein a carbon-containing compound gaswithout an unsaturated bond is flowed onto the catalyst raw materialand/or the catalyst particles in at least one of the catalyst particleforming step and the carbon nanotube synthesizing step.
 2. The method ofproducing carbon nanotubes according to claim 1, wherein thecarbon-containing compound gas without an unsaturated bond is flowedonto the catalyst particles at least in the carbon nanotube synthesizingstep.
 3. The method of producing carbon nanotubes according to claim 1,wherein the carbon-containing compound gas without an unsaturated bondis a saturated hydrocarbon gas.
 4. The method of producing carbonnanotubes according to claim 3, wherein the saturated hydrocarbon gas ismethane.
 5. The method of producing carbon nanotubes according to claim1, wherein the raw material gas contains acetylene or a gas generatingacetylene in a reactor.
 6. The method of producing carbon nanotubesaccording to claim 1, wherein the carbon-containing compound gas withoutan unsaturated bond is flowed onto the catalyst raw material and/or thecatalyst particles at least in the catalyst particle forming step, andthe method further comprises a releasing step of releasing the catalystparticles from the heated state between the catalyst particle formingstep and the carbon nanotube synthesizing step.
 7. Carbon nanotubesproduced by the method of producing carbon nanotubes according to claim1, wherein a ratio G/D determined by Raman spectroscopy is 10 or more.